The invention relates to a method for operating an internal combustion engine with an exhaust gas purifying device, wherein the exhaust gas purifying device has a catalytic converter through which an exhaust gas stream of the internal combustion engine can be conducted and a first lambda probe arrange upstream of the catalytic converter in the exhaust gas stream and a second lambda probe arranged in the exhaust gas stream arranged upstream of the catalytic converter, wherein as first lambda probe (5) a broadband lambda probe and as a second lambda probe (6) a binary lambda probe is used. The invention also relates to an internal combustion engine.
The method serves for operating the internal combustion engine or respectively the exhaust gas purifying device assigned to the internal combustion engine. Exhaust gas, which is exhausted by the internal combustion engine and generated during the combustion of fuel is at least partially purified of pollutants by means of the exhaust gas purifying device. For this purpose the exhaust gas purifying device has at least one catalytic converter through which exhaust gas of the internal combustion engine can be conducted in the form of the exhaust gas stream. Further, two lambda probes are assigned to the exhaust gas purifying device, wherein the first lambda probe is arranged upstream of the catalytic converter and the second lambda probe is arranged downstream of the catalytic converter, so that the oxygen content of the exhaust gas can be determined at the respective position upstream or downstream of the catalytic converter. For this purpose the first and the second lambda probe protrude into the exhaust stream. The first lambda probe provides a first lambda signal and the second lambda probe a second lambda signal, wherein a first lambda value can be determined from the first lambda signal and a second lambda value can be determined from the second lambda signal.
The catalytic converter has an oxygen storage or respectively operates as such. This means that when a lean exhaust gas is present—i.e., in the case of an oxygen excess at combustion of λ greater than one—oxygen transitions from the exhaust gas into the oxygen accumulator and is intermittently stored therein. On the other hand when a rich exhaust gas—resulting from the combustion with fuel excess with λ smaller than one—is present, oxygen is removed from the oxygen accumulator. This ensures at least over a certain period of time that the stoichiometric ratio with λ=1 required for the purification of exhaust gas can at least approximately be provided. Correspondingly the quality of the catalytic converter can for example be determined by way of the oxygen storage capacity. Preferably the oxygen storage capacity is determined periodically.
In particular the lambda probe arranged upstream of the catalytic converter often only has a low accuracy. For example the first lambda signal provided by the first lambda probe deviates by a defined value, the so-called offset, from the conditions actually existing in the exhaust gas at the site of the first lambda probe. Due to this error it may occur that the internal combustion engine is set to a composition of a fuel air mixture supplied to the internal combustion engine, which deviates from the one that would be required to achieve a good or better converting efficiency in the catalytic converter. Correspondingly the goal is to compensate the error of the first lambda probe or the offset as quickly as possible. This can for example be accomplished by means of a controller, which regulates the signal provided by the second lambda probe to a lambda target value. This regulation however can only be conducted with a very slow regulation speed because when using a higher regulation speed regulation oscillations occur which in turn themselves lead to a poorer converting efficiency of the catalytic converter.